The invention relates to the field of controllers for blood treatment devices and systems that withdraw and infuse blood from patients. The invention is particularly suitable for blood filtration systems which are coupled to patients for several hours during each treatment. The invention also relates to controllers for medical devices that distinguish and react appropriately to minor device difficulties that may be cured automatically or by the patient, and to more serious difficulties that require the attention of a nurse or other medical professional.
There are a number of medical treatments, such as ultrafiltration, apheresis and dialysis, that require blood to be temporarily withdrawn from a patient, treated and returned to the body shortly thereafter. While the blood is temporarily outside of the body, it flows through an xe2x80x9cextracorporeal blood circuitxe2x80x9d of tubes, filters, pumps and/or other medical components. In some treatments, the blood flow is propelled by the patient""s blood pressure and gravity, and no artificial pump is required. In other treatments, blood pumps provide additional force to move the blood through the circuit and control the flow rate of blood through the circuit. These pumps may be peristaltic or roller pumps, which are easy to sterilize, are known to cause minimal clotting and damage to the blood cells, and are inexpensive and reliable.
Brushed and brushless DC motors are commonly used to rotate peristaltic pumps. A motor controller regulates the rotational speed of blood pumps. The speed of a pump, expressed as rotations per minute (RPM), regulates the flow rate of the blood through the circuit. Each revolution of the pump moves a known volume of blood through the circuit. The blood flow rate through the circuit can be easily derived from the pump speed. Accordingly, the pump speed provides a relatively accurate indicator for the volume flow of blood through an extracorporeal circuit.
Existing blood pump controllers include various alarms and interlocks that are set by a nurse or a medical technician (collectively referred to as the operator), and are intended to protect the patient. In a typical dialysis apparatus, the blood withdrawal and blood return pressures are measured in real time, so that sudden pressure changes are quickly detected. Sudden pressure changes in the blood circuit are treated as indicating an occlusion or a disconnect in the circuit. The detection of a sudden pressure change causes the controller to stop the pump and cease withdrawal of blood. The nurse or operator sets the alarm limits for the real time pressure measurements well beyond the expected normal operating pressure for the selected blood flow, but within a safe pressure operating range.
Existing controllers do not distinguish between minor blood pump problems that can be safely and easily solved automatically by the controller or by the patient, and more serious problems that require a nurse or other medical professional to attend to the patient and blood circuit. For example, existing controllers typically stop their pumps and issue alarms, upon detection of a partial occlusion in the blood circuit. In response to each alarm of an occlusion in the blood circuit, a nurse attends to the patient, inspects the blood pump and associate catheters, and restarts the pump. Until the nurse restarts the blood pump, the filtration treatment is being delayed.
Partial occlusions in a blood circuit are relatively common occurrences. Nurses frequently have to attend to patients and extracorporeal blood circuits to correct partial occlusions. The delay in restarting the blood pump extends and exacerbates the blood treatment, which may be a period of several hours. The frequent alarms for partial occlusions increase the workload on nurses and the amount of time that they must devote to an individual patient undergoing ultrafiltration treatment.
U.S. Pat. No. 4,227,526 describes a home-treatment dialysis machine that issues audio instructions to the patient on how to correct certain malfunctions, including excessive pressure in the extracorporeal blood circulation circuit. This device is intended for use at home, where there is no nurse or other medical professional present. The dialysis machine disclosed in the ""526 Patent does not discriminate between minor dialysis malfunctions that should be treated by the patient, and more serious malfunctions that require treatment by a nurse. U.S. Pat. No. 6,026,684 describes a blood drawing apparatus that detects low blood flow in the blood withdrawal catheter and prompts a patient to restore blood flow by squeezing a hand gripper. The device disclosed in the ""684 Patent also does not discriminate between minor occlusion problems and more serious problems. In addition, the devices disclosed in the ""526 Patent and in the ""684 Patent do not allow a patient to differentiate between withdrawal and infusion lines of a blood circuit. With the devices disclosed in the ""526 and ""684 Patents, a nurse is not informed as to serious problems, and with minor occlusion difficulties there is no indication as to whether the difficulty has arisen in the withdrawal or infusion catheters, which are generally inserted in different arms of the patient.
There is a long-felt need for controllers for an extracoporeal blood circuit that discriminates between minor difficulties that can be cured automatically or by prompting the patient to take corrective action, and more serious problems that require the attention of a nurse or other medical professional. For example, there is a need for a controller for an extracorporeal blood circuit that can automatically reacts to partial occlusions in a blood withdrawal or infusion catheter or prompt the patient to move his arm or body to alleviate the occlusion. It may be advantageous for the controller to distinguish between minor difficulties in the blood circuit, such as partial occlusions, and more serious problems, such as total occlusions or extended partial occlusions. For more serious problems, the controller may issue an alarm to a nurse. There is also a need for a blood treatment controller that identifies for a patient a particular arm (or other body part) to be moved so as to alleviate a partial occlusion in a withdrawal or infusion catheter.
A novel blood withdrawal system has been developed that enables rapid and safe recovery from occlusions in a withdrawal vein without participation of an operator, loss of circuits to clotting, or annoying alarms. A controller has been developed that compensates for and remedies temporary vein collapse during blood withdrawal or infusion. Not all episodes of a vein collapse require intervention from a doctor or nurse, and do not require that blood withdrawal ceased for an extended period. For example, vein collapse can temporarily occur when the patient moves or a venous spasm causes the vein to collapse in a manner that is too rapid to anticipate and temporary. There has been a long-felt need for a control system for an extracorporeal circuit that can automatically recover from temporary occlusions. The controller may also temporarily stops blood withdrawal when vein collapse occurs and, in certain circumstances, infuses blood into the collapsed vein to reopen the collapsed vein. Further, the controller may stop or slow filtration during periods of reduced blood flow through the blood circuits so as to prevent excessive removal of liquids from the blood of a patient. Moreover, the controller may prompt a patient to move an arm or his body to alleviate a partial occlusion in a withdrawal or infusion vein.
In response to occlusion blood and ultrafiltrate pump rates are reduced automatically. If occlusion is removed, these flow rates are restored immediately and automatically. The patient is prompted to move, if the occlusion persists for more than a few seconds. The operator is alarmed if occlusions are prolonged or frequent. An alarm is canceled automatically if the occlusion is alleviated, and blood and ultrafiltrate flows are restored.
Peripheral vein access presents unique problems that make it difficult for a blood withdrawal controller to maintain constant flow and not to create hazards for the patient. These problems are unlike those encountered with conventional dialysis treatments that rely on a surgically created arterio-venous shunt or fistula to withdraw blood and are administered in controlled dialysis centers. Using the present controller, for example, a patient may stand up during treatment and thereby increase the static pressure head height on the infusion side resulting in a false occlusion. The controller adjusts the blood flow rate through the extracorporeal circuit to accommodate for pressure changes. As the patient rises each centimeter (cm), the measured pressure in the extracorporeal circuit may increase by 0.73 mm Hg (milliliter of mercury). A change in height of 30 cm (approximately 1 ft) will result in a pressure change of 21 mm Hg. In addition, the patient may bend his/her arm during treatment and, thereby, reduce the blood flow to the withdrawal vein. As the flow through the withdrawal catheter decreases, the controller reduces pump speed to reduce the withdrawal pressure level. Moreover, the blood infusion side of the blood circulation circuit may involve similar pressure variances. These infusion side pressure changes are also monitored by the controller which may adjust the pump to accommodate such changes.
The controller may be incorporated into a blood withdrawal and infusion pressure control system which optimizes blood flow at or below a preset rate in accordance with a controller algorithm that is determined for each particular make or model of an extraction and infusion extracorporeal blood system. The controller is further a blood flow control system that uses a real time pressure measurement as a feedback signal that is applied to control the withdrawal and infusion pressures within flow rate and pressure limits that are determined in real time as a function of the flow withdrawn from peripheral vein access.
The controller may govern the pump speed based on control algorithms and in response to pressure signals from pressure sensors that detect pressures in the blood flow at various locations in the extracorporeal circuit. One example of a control algorithm is a linear relationship between a minimum withdrawal pressure and withdrawal blood flow. Another possible control algorithm is a maximum withdrawal flow rate. Similarly, a control algorithm may be specified for the infusion pressure of the blood returned to the patient. In operation, the controller seeks a maximum blood flow rate that satisfies the control algorithms by monitoring the blood pressure in the withdrawal tube (and optionally in the infusion tube) of the blood circuit, and by controlling the flow rate with a variable pump speed. The controller uses the highest anticipated resistance for the circuit and does not adjust flow until this resistance has been exceeded. If the maximum flow rate results in a pressure level outside of the pressure limit for the existing flow rate, the controller responds by reducing the flow rate, such as by reducing the speed of a roller pump, until the pressure in the circuit is no greater than the minimum (or maximum for infusion) variable pressure limit. The controller automatically adjusts the pump speed to regulate the flow rate and the pressure in the circuit. In this manner, the controller maintains the blood pressure in the circuit within both the flow rate limit and the variable pressure limits that have been preprogrammed or entered in the controller.
In normal operation, the controller causes the pump to drive the blood through the extracorporeal circuit at a set maximum flow rate. In addition, the controller monitors the pressure to ensure that it conforms to the programmed variable pressure vs. flow limit. Each pressure vs. flow limit prescribes a minimum (or maximum) pressure in the withdrawal tube (or infusion tube) as a function of blood flow rate. If the blood pressure falls or rises beyond the pressure limit for a current flow rate, the controller adjusts the blood flow by reducing the pump speed. With the reduced blood flow, the pressure should rise in the withdrawal tube (or fall in the return infusion tube). The controller may continue to reduce the pump speed, until the pressure conforms to the pressure limit for the then current flow rate.
When the pressure of the adjusted blood flow, e.g., a reduced flow, is no less than (or no greater than) the pressure limit for that new flow rate (as determined by the variable pressure vs. flow condition), the controller maintains the pump speed and operation of the blood circuit at a constant rate. The controller may gradually advance the flow rate in response to an improved access condition, provided that the circuit remains in compliance with the maximum rate and the pressure vs. flow limit.
The controller has several advantages over the prior art including (without limitation): that the controller adjusts the pump speed to regulate the blood flow rate and maintain the blood pressure within prescribed limits, without requiring the attention of or adjustment by an operator; the controller adjusts blood flow in accordance with an occlusion pressure limit that varies with flow rate; the controller adaptively responds to partial occlusions in the withdrawal blood flow, and the controller prompts the patient to move a particular arm or move his body to alleviate partial occlusions in a withdrawal or infusion line. In addition, the controller discriminates between minor problems with the blood flow, such as a partial occlusion, that may it may automatically respond to by reducing pump speed or by prompting the patient to move an arm, and more serious problems, such as prolonged or excessive occlusions, that require an alarm to call for a nurse. Moreover, the controller may suspend or slow the rate of removal of filtrates from the blood during periods of reduced blood flow through the blood circuit. Further, the controller implements other safety features, such as to detect the occurrence of total unrecoverable occlusions in the circuit and disconnections of the circuit, which can cause the controller to interpret that blood loss is occurring through the extracorporeal circuit to the external environment and stop the pump.
In a first embodiment, the invention is a method for controlling blood flow through an extracorporeal blood circuit having a controller comprising the steps of: withdrawing the blood from a withdrawal blood vessel in a patient into the extracorporeal circuit, treating the blood in the circuit and infusing the treated blood into the patient; detecting an occlusion which at least partially blocks the withdrawal or infusion of the blood; in response to the detection of the occlusion, the controller automatically prompts the patient to move to alleviate the occlusion, and in response to a prolonged occlusion, the controller issues an alarm.
In a second embodiment, the invention is a method for controlling blood flow through an extracorporeal blood ultrafiltration circuit having a controller comprising the steps of: (a) selecting a desired filtration rate for the ultrafiltration circuit to extra filtrate for an ultrafiltration treatment; (b) withdrawing the blood from a withdrawal blood vessel in a patient into the extracorporeal circuit, filtering the blood to extract filtrates at the desired filtration rate, and infusing the filtered blood into the patient; (c) detecting a pressure of the blood being withdrawn or infused beyond a predetermined threshold pressure value; (d) reducing a blood flow rate through the circuit in response to the detection of the variation in pressure; (e) reducing a rate of filtrate extraction to a rate less than the desired filtration rate and no greater than twenty percent of a rate of blood flow through the circuit; (f) increasing the blood flow rate through the circuit after determining that the pressure of the blood being withdrawn or infused is within the threshold pressure value, and (g) increasing the filtration rate after step (e).